The basic geographical unit used in carrying out both the 1901 and 1911 censuses is the District Electoral Division, a subdivision of the county, used, as the name implies, for electoral purposes. To use the returns, ideally the relevant street or townland should be known. The 1901 Townlands Index, based on the census returns, supplies the name and number of the D.E.D. in which the townland is situated.

County by county volumes, on open shelves in the National Archives< of Ireland Reading Room, go through the District Electoral Divisions in numerical order, for both 1901 and 1911, giving the name and number of each of the townlands they contain.

To order the returns for a specific townland, it is necessary to supply the name of the county, the number of the District Electoral Division and the number of the townland, as given in these volumes.

For the cities of Belfast, Cork, Dublin and Limerick, separate street indexes have been compiled, and are also on open shelves in the Reading Room.
The street or part of a street is numbered, and you need the number to order specific returns. Between 1901 and 1911, some changes took place in the District Electoral Divisions, and their numbering is different in some cases. There is no separate townlands index for 1911, but the changes are minor, so that a District Electoral Division numbered 100 in 1901 may be 103 in 1911, and can be found simply by checking the Divisions above and below 100 in the 1911 volume for the county.

The returns for 1901 have been bound into large volumes, while those for 1911 are still loose and in boxes. The 1901 returns are now also available on microfilm. In each case, all the returns for a townland or street are grouped together and preceded by an enumerator's abstract which gives the details of the houses and lists the names of the heads of households.

Again, this can be a useful way of finding a location for a family.

One problem which can arise in searching a large area is the difficulty of translating from the earlier geographical division of a parish, for instance, to the relevant District Electoral Divisions, since these latter cut across the earlier boundaries. The most straightforward, though cumbersome, way to cover a large area is to take all the townlands in particular civil parish and check their District Electoral Divisions in the 1901 Townlands Index.

The 1841 Townlands Index, also known as addenda to the 1841 Census, and available on request from the National Archives Reading Room staff or in the National Library (Ir 310 c 1), organizes townlands alphabetically within civil parishes.